US2794416A

US2794416A - Apparatus for controlling charged particles

Info

Publication number: US2794416A
Application number: US371279A
Authority: US
Inventors: James B Shepherd
Original assignee: Ransburg Corp
Current assignee: Ransburg Corp
Priority date: 1953-07-30
Filing date: 1953-07-30
Publication date: 1957-06-04
Anticipated expiration: 1974-06-04

Description

June 4, 1957 J. B. SHEPHERD 2,794,416

APPARATUS FOR CONTROLLING CHARGED PARTICLES Filed July 30, 1953 JNVEN TOR.

JAMES B. SHEPHERD 2,794,416 Patented June 4, 1957 APPARATUS FQR CONTROLLING CHARGED PARTICLES James B. Shepherd, Indianapolis, Ind, assignor to Ransburg Electra-Coating Corp., Indianapolis, Ind., a corporation of Indiana Application July 30, 1953, Serial No. 371,279

9 Claims. (Cl. 118-51) My invention has to do with the electrostatic charging of particles of coating material and more particularly with improvements in apparatus for controlling the path of movement of such charged particles.

The electrostatic charging of particles in order to efiect the deposition of the particles on articles is now in wide use in industry. Particles, either finely divided solids or sprays of liquid material, are introduced into an electrostatic field from a source remote from the article on which they are to be deposited, are given a high electrical potential in relation to such articles and thereby attracted toward and deposited on the surfaces of the article. One efiicient commercial type of electrostatic coating with atomized, charged particles utilizes an atomizing head and a grounded conveyor which moves the articles in succession along a predetermined path spaced from the aforementioned atomizing head. Liquid coating material is fed to the head which is connected to a source of high voltage to establish an electrostatic field between the head and the articles which are grounded through the conveyor. Finely divided particles of the coating material are atomized from the edge of the head and projected into the coating zone existing between the atornizing head and the articles. The particles are thus attracted to and deposited on the articles under the influence of the electrostatic field.

In electrostatic coating the finely divided particles are deposited generally in proportion to the lines of electrical force existing over the surface of the article, so long as the atmosphere adjacent the article does not become ionizing. The electrostatic coating apparatus and the articles are normally so arranged that the great majority of the lines of force will terminate on surfaces of articles in the coating zone. However, since the articles are often maintained at their predetermined electrical potential through electrical connection to the conveyor which is utilized primarily to support and guide the articles through the coating zone, the conveyor itself is generally at the same potential (normally ground potential) as the articles themselves. Therefore, particularly When the distance between the conveyor structure adjacent the coating zone and the articles therein is not too great it will be seen that some of the lines of force may terminate on adjacent portions of the conveyor itself. In addition, various conditions which may from time to time occur in the operation of an electrostatic coating system may cause stray particles of coating material to occasionally escape from the coating zone existing between the charging means and the articles and thereupon to be attracted toward and deposited on an adjacent portion of the conveyor structure.

While the percentage of the total particles deposited on conveyor structure may be very small, some parts of the structure do not move as do the articles and the article supports but remain fixed adjacent the coating zone. These fixed parts and in some situations even moving parts may tend to accumulate an undesirable and unwanted deposit of the particled material. The continued deposition of coating material on portions of the conveyor system adjacent the coating zone may in extreme cases cause work stoppages and in overhead conveyors result in accumulated coating material dropping onto articles moving along the conveyor. In any event, such deposition necessitates frequent cleaning of the conveyor and represents a loss both in overall efficiency and in the percentage of the coating material being deposited on the articles.

In the past physical shielding ofthe structure adjacent a coating zone has been attempted by substantially wrapping or covering the structure to be protected as by grounded metal sheeting. Such procedure merely transfers the unwanted deposit from one surface to another and in many instancesfth'e use of a physical shielding member presents objectionable interference to the electrical characteristics of the electrostatic coating zone. Separate secondary electrodes at the high voltage of a discharge electrode have also been tried but such electrodes could not be placed near the conveyor structure due to electrical interference with the electrostatic field in the coating zone and also sparking or electrical discharge from the secondary electrodes to adjacent conveyor portions resulted.

I have discovered that the deposition of material on portions of the conveying system or other structure adjacent an electrostatic coating zone can be eliminated or at least substantially reduced by the use of a properly designed and spaced electrically shielding means. Such a shielding means may also be used to change the direction of particles moving at the periphery of the coating zone and to assist in depositing these particles on surfaces of the articles to be coated such as the tops and bottoms of containers moving through a coating zone. In its preferred form such a means is extremely simple and easy to install, requires little cleaning, and avoids the dangers of sparking inherent in previous apparatus used in attempting to prevent deposition of coating material on structure in the vicinity of the coating zone.

In one convenient embodiment of my invention the shielding means constitutes an electrode having 'a relatively small surface area such as a wire maintained at an electrical potential of the same sign as the discharge electrode frequently used in electrostatic coating systems and may be electrically connected thereto. The shielding electrode may be a cable or an insulation-covered metal wire and in such later case the insulated wire may be brought extremely close to the conveyor structure and positioned at least partly between the structure to be shielded and the discharge electrode without danger of sparking or interference with the electrical properties of the coating zone proper. Such a shielding means creates an effective high potential zone of limited extent about the shielding means which will act to electrically repel stray particles of coating material which would normally be deposited on the shielded structure and to redirect the particles toward and deposit them on articles in the coating zone. In another embodiment of my invention a noninsulated member or members which constitute the shielding means is maintained at a less potential than the discharge electrode but of the same sign.

It is, therefore, an object of my invention to provide improvements in the control of electrostatically charged particles.

Another object of my invention is to provide novel means to inhibt the deposition of electrically charged particles on predetermined surfaces. I

A further object is to provide a simple and economical arrangement to prevent the deposition of coating material particles on portions of a conveying system adjacent an electrostatic coating zone.

A still further object is to, provide a charged particlecontrolled zone outside the coating zone proper for redirecting stray coating material particles into the coating The scope of the invention will be pointed out in the appended claims.

Referring to Figs. 1 and 2 the articles to be coated (here illustrated as cylindrical containers are carried by hangers 11 which are hung from rotors 12 which in turn are suspended from a series of pendants 14. Pendants 14 are hung at substantially equally spaced intervals along a conveyor chain 15 which is moved by means not shown along a conveyor track 16 in a predetermined path which passes through the coating zone. Rotators 12 are arranged to be rotated, and thereby to transmit rotation through hangers II to articles 10, by virtue of engagement of the outer cylindrical surface of the rotator with a rotator bar 17 as shown. The articles are thus traversed along the path of conveyor track 16 and rotated during such movement to increase the uniformity of the distri- 'bution'of coating material over the surfaces of the containers. The articles are grounded by electrical contact with the grounded conveyor.

Adjacent and to one side of the conveyor-borne articles is a three-head airless atomizing arrangement for the coating operation, although itwill be understood that in practice other atomizing and/ or coating systems such as those employing compressed air as the principle atomizing means might well be used with my invention. The main coating arrangement illustrated comprises stands 20 and 21 from which rise

vertical columns

22 and 23 in turn carrying between them an inclined support member 24 on which are mounted three bell-shaped rotating atomizing heads designated as 25, 26 and 27, each adapted to be rotated through suitable drive connections by a motor 28. The atomizing heads may be of different diameters as illustrated and suitably spaced to secure the optimum uniformity of distribution of coating material on containers 10.

The three heads which serve as discharge electrodes are raised to the desired electrical potential as by electrical connection to a cable 29 which serves as the hot terminal of a high voltage power pack 30, grounded as shown. Atomizing

heads

25, 26 and 27 are suitably supplied with liquid coating material, as by

hoses

31, 32 and 33 from a pump 34 drawing liquid coating material from'a reservoir not shown. Articles 10 are grounded through the conveying system and in one suitable arrangement the heads are spaced approximately 12" from the nearest surface of moving articles 10 to obtain a potential difference of approximately 100,000 volts between the heads and articles 10. Operation of this arrangement will result in atomization of the liquid coating material from the heads as a spray of finely divided liquid particles, raising the particles to the potential of the discharge electrodes (in this case atomizing

heads

25, 26,

and 27), dispersion of the particles, and movement of the particles through the quiescent air of the coating zone terminate on parts of the conveying system, including ing Zone, and shielding means maintained at an electrical rotors i2 and pendants 14 rather than on the articles which serve as the terminus of the great majority of the lines of force. Moreover, other factors in the operation of the coating system may permit the escape of a small number of particles from the coating zone proper, which particles may become deposited on some part of the conveying structure adjacent the coating zone.

To prevent or at least inhibit lines of force'from terminating on the conveyor structure and otherwise to prevent the deposition of coating particles thereon, there is provided a shielding means which is shown in the form of a cable 35 having a core of copper wire of approximately 0.10 inch diameter covered with a polyethylene type of insulating sheath having an outside diameter of three-eighths of an inch. The ends of cable 35 are connected to the housings of heads 25 and 27 so that the copper core of the cable will be raised to substantially the potential of

heads

25, 26 and 27. Cable 35 is arranged generally above the atomizing heads so that it presents three

parallel segments

35a, 35b, and 350 connected in series and located in shielding relation to track 16, pendants 14, and rotors 12 and rotator bar 17 respectively of the conveyor system adjacent the coating zone. Segment 350 is located substantially between the atomizing heads on one hand and rotors 12 and rotator bar 17 on the other. Cable 35 may be held in position by connection ,to one or more insulating supports such as bars 36 and hooks 37 as shown.

In the operation of the coating system embodying my novel shielding means in the form shown the high potential at which the copper core of

segments

35a, 35b, and 35c is subjected will create a zone about each segment of the same electrical sign as that of the atomized particles. The zones will, however, be restricted in their area due to the insulating efiect of the polyethylene sheath so that there will be established a composite zone of limited radial extent adjacent the conveyor structure and at least in part between said structure and atomizing

heads

25, 26 and 27 serving to effectively shield conveyor structure adjacent the coating zone and at the same time to repel stray particles of coating material from the

zones surrounding segments

35a, 35b, and 350 and to redirect particles toward the coating zone for deposition on articles 10. The insulating sheath by radially restricting the eifective zone of high potential about cable 35 permits the shielding means to lie closely adjacent the conveyor within what would otherwise be sparking distance between the charged copper wire core and the grounded conveyor.

The use of a flexible shielding means such as the cable shown permits the shielding means to be readily adapted to variations in structure as found in various types of conveying systems. My invention is not, of course, limited to aninsulation-covered cable and in certain instances elimination of the insulation may be desired. However,

in such cases it may be desirable to maintain the shielding means at a lower potential but at the same sign as the discharge electrode.

I claim: a

1. In an electrostatic system for depositing charged particles of coating material on an article, said system having a coating zone lying substantially between the article to be coated and a discharge electrode maintained at a high electrical potential in relation to the article, means including an atomizer for projecting into said coating zone a spray of charged liquid particles of coating material, a conveyor for supporting and moving the article through the coating zone along a path spaced from said atomizer, means for maintaining said conveyor at a particle-attracting electrical potential, said conveyor having portions adjacent the coating zone which in the absence of a shielding means would attract and accumulate particles of coating material escaping from the coatpotential in relation to said conveyor and of the same sign as the discharge electrode, said shielding means having portions located adjacent the particle-attracting portions of the conveyor and closer thereto than to the coating zone.

2. Apparatus as set forth in claim 1 wherein the shielding means comprises a flexible metal core electrically connected to said discharge electrode and a sheath of dielectric material immediately surrounding said core.

3. In an electrostatic system for depositing charged spray particles of liquid coating material on articles having a coating zone between articles to be coated and a discharge electrode maintained at a high potential, means including an atomizer for projecting into said coating zone a spray of liquid particles of coating material to be charged by said discharge electrode, a conveyor for supporting and moving the article through the coating zone along a predetermined path spaced from said atomizer, means for maintaining said conveyor and the articles at substantially ground potential, said conveyor having portions adjacent the coating zone which in the absence of shielding means would attract and accumulate spray particles of coating material escaping from the coating zone, and shielding means comprising a flexible metal member electrically connected to the discharge electrode having a portion lying adjacent to said coating zone and a dielectric member interposed between said flexible metal member and said conveyor to repel spray particles and prevent their deposition on portions of the conveyor adjacent the coating zone.

4. Electrostatic apparatus for coating the surface of an article comprising a discharge electrode, a support for supporting an article having a surface spaced from said discharge electrode, means having a particle-attracting surface spaced adjacent to the article surface but further from said discharge electrode than is the article surface, means including a high voltage source for establishing a high electrical potential difference between said discharge electrode on the one hand and the article surface particle-attracting surface on the other, means for forming a spray of particled material at a point near said discharge electrode, said particles having substantially the potential of said discharge electrode, a shielding member of relatively small surface area having a portion located between said particle-attracting surface and the discharge electrode but closer to said particleattracting surface than to said discharge electrode, and means for maintaining said shielding member at substantially the electrical potential of the discharge electrode.

5. Electrostatic apparatus for coating the surface of an article comprising a discharge electrode, a support for supporting an article having a surface spaced from said discharge electrode, means having a particle-attracting surface spaced adjacent to the article surface but further from said discharge electrode than is the particle surface, means including a high voltage source for establishing a high electrical potential difierence between said discharge electrode on the one hand and the article surface and said particle-attracting surface on the other, means for forming a spray of particled material at a point near said discharge electrode, said particles having substantially the potential of said discharge electrode, a shielding member having a portion positioned between said particle attracting surface and the discharge electrode but closer to said particle-attracting surface than to said discharge electrode, and means for maintaining said shielding member at a difference of potential from the surfaces and of the same sign as the discharge electrode.

6. Electrostatic apparatus for coating an article comprising means for forming a spray of finely divided particles at a region spaced from the article to be coated, a surface adjacent the article but further from the region of spray formation than is the article, means including a high voltage source for establishing a particle-attracting electrical potential diiference between the article and said surface on one hand and said spray particles on the other, shielding means of relatively small surface area having portions lying between said surface and the point of spray formation but closer to said surface than to said point, and means for maintaining said shielding means at substantially the electrical potential of the spray particles.

7. Electrostatic apparatus for coating an article comprising a conveyor for supporting and moving the article over a predetermined path in a coating zone, means spaced from the conveyor and the supported article for discharging a spray of charged particles into the coating zone for deposition on the article, means for maintaining the conveyor at a spray particle-attracting potential, and shielding means maintained at substantially the same potential as that of the charged spray particles, said shielding means being spaced from both the spray discharging means and the conveyor and positioned within the outer limits of the coating zone closer to the conveyor than to the article to inhibit the deposition of spray particles on the conveyor.

8. Electrostatic apparatus for coating an article comprising a support for supporting the article in a coating zone, means including an atomizer spaced from the support and the supported article for discharging a spray of charged particles into the coating zone for deposition on the article, means for maintaining the support and the supported article at a different electrical potential from the charged spray, shielding means disposed between the atomizer and the support and having portions lying closer to said support than to said atomizer, said shielding means comprising a conducting member surrounded by an insulating member, and means for maintaining at least a portion of said shielding means at substantially the potential of the charged spray.

9. Electrostatic apparatus for coating an article comprising a conveyor for supporting and moving an article over a predetermined path in a coating zone, means spaced from the conveyor and the supported article for discharging a spray of finely divided particles into the coating zone for deposition on the article, means including a source of high voltage having one terminal connected to said spray discharging means for charging the spray particles and for maintaining between said spray discharging means on the one hand and the conveyor and the article on the other a depositing electrostatic field, and shielding means maintained at substantially the same potential as said spray discharging means, said shielding means positioned within the outer limits of the depositing electrostatic field and closer to the conveyor than to the article to inihibit the deposition of spray particles on the conveyor.

References Cited in the file of this patent UNITED STATES PATENTS 1,855,869 Pugh Apr. 26, 1932 2,191,827 Benner et a1. Feb. 27, 1940 2,466,906 Miller et a1. Apr. 12, 1949